Model Surface Studies

In recent years, there has been a strong effort to use model catalyst surfaces combined with atomistic modeling approaches to understand why Ft alloys show activity enhancements and to predict new surfaces fhaf show even greater activity for oxygen reduction. 

The study of model Ff alloy surfaces has been pioneered by the work of Ross and Markovic in fhe Unifed Sfafes, firsf af fhe Lawrence Berkeley National Laboratory and more recently at Argorme National Laboratory. A series of bulk FfgM (M = V, Ti, Fe, Co, Ni) alloy polycrysfalline maferials were prepared and fheir surfaces characferized by surface science fechniques. The composition of fhe surface was found to be dependenf on posftieatment. Surfaces sputtered with Ar ions showed compositions similar to the bulk, while surfaces fhaf had been annealed in vacuum showed stiong segregation of Ft to give Ft skins. 

The investigation of model surfaces for OR has been closely followed by theoretical calculations with the aim to develop robust thermodynamic 

Oxygen reduction specific activities of the low index planes of Pt and PtjNi in 0.1 M HCIO4 at 0.9 V versus RHE at 60°C. (From Science, 315, 493 (2007). Reproduced with permission from AAAS.) 

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Wu M-C, Estrada C A, Corneille J S and Goodman D W 1996 Model surface studies of metal oxides adsorption of water and methanol on ultrathin MgO films on Mo(IOO) J. Chem. Phys. 96 3892... 

SAMs are generating attention for numerous potential uses ranging from chromatography [SO] to substrates for liquid crystal alignment [SI]. Most attention has been focused on future application as nonlinear optical devices [49] however, their use to control electron transfer at electrochemical surfaces has already been realized [S2], In addition, they provide ideal model surfaces for studies of protein adsorption [S3]. 

As described in the chapter on band structures, these calculations reproduce the electronic structure of inhnite solids. This is important for a number of types of studies, such as modeling compounds for use in solar cells, in which it is important to know whether the band gap is a direct or indirect gap. Band structure calculations are ideal for modeling an inhnite regular crystal, but not for modeling surface chemistry or defect sites. 

The properties of the periodic surfaces studied in the previous sections do not depend on the discretization procedure in the hmit of small distance between the lattice points. Also, the symmetry of the lattice does not seem to influence the minimization, at least in the limit of large N and small h. In the computer simulations the quantities which vary on the scale larger than the lattice size should have a well-defined value for large N. However, in reality we work with a lattice of a finite size, usually small, and the lattice spacing is rather large. Therefore we find that typical simulations of the same model may give diffferent quantitative results although quahtatively one obtains the same results. Here we compare in detail two different discretization... 

Knowledge about protein folding and conformation in biological systems can be used to mimic the design of a desired nanostructure conformation from a particular MBB and to predict the ultimate conformation of the nanostructure [152]. Such biomimetic nano-assembly is generally performed step by step. This wiU allow observation of the effect of each new MBB on the nanostructure. As a result, it is possible to control accurate formation of the desired nanostmcture. Biomimetic controlled and directed assembly can be utilized to investigate molecular interactions, molecular modeling, and study of relationships between the composition of MBBs and the final conformation of the nanostmctures. Immobilization of molecules on a surface could facilitate such studies [153]. 

BB-SFG, we have investigated CO adsorption on smooth polycrystaHine and singlecrystal electrodes that could be considered model surfaces to those apphed in fuel cell research and development. Representative data are shown in Fig. 12.16 the Pt nanoparticles were about 7 nm of Pt black, and were immobilized on a smooth Au disk. The electrolyte was CO-saturated 0.1 M H2SO4, and the potential was scanned from 0.19 V up to 0.64 V at 1 mV/s. The BB-SFG spectra (Fig. 12.16a) at about 2085 cm at 0.19 V correspond to atop CO [Arenz et al., 2005], with a Stark tuning slope of about 24 cm / V (Fig. 12.16b). Note that the Stark slope is lower than that obtained with Pt(l 11) (Fig. 12.9), for reasons to be further investigated. The shoulder near 2120 cm is associated with CO adsorbed on the Au sites [Bhzanac et al., 2004], and the broad background (seen clearly at 0.64 V) is from nomesonant SFG. The data shown in Figs. 12.4, 12.1 la, and 12.16 represent a hnk between smooth and nanostructure catalyst surfaces, and will be of use in our further studies of fuel cell catalysts in the BB-SFG IR perspective. 

Henry CR. 1998. Surface studies of supported model catalysts. Surf Sci Rep 31 235-325. Henry C. 2003. Adsorption and reaction at supported model catalysts. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker. 

The protein-containing colloidal solutions of water-in-organic solvents are optically transparent. Hence, absorption spectroscopy, circular dichroism spectroscopy and fluorescence spectroscopy are found to be convenient for studying biocatalysis [53]. The reversed micelles are interesting models for studying bioconversion, since the majority of the enzymes in vivo act inside or on the surface of biological membranes. 

The ability of STM to image at the atomic scale is particularly exemplified by the two other chapters in the book. Thornton and Pang discuss the identification of point defects at Ti02 surfaces, a material that has played an important role in model catalyst studies to date. Point defects have been suggested to be responsible for much of the activity at oxide surfaces and the ability to identify these features and track their reactions with such species as oxygen and water represents a major advance in our ability to explore surface reactions. Meanwhile, Baddeley and Richardson concentrate on the effects of chirality at surfaces, and on the important field of surface chirality and its effects on adsorption, in a chapter that touches on one of the fundamental questions in the whole of science - the origins of life itself ... 

One of the most promising tools in the study of the nature and structure of adsorbed molecules is photoelectron spectroscopy (40), and results from such experiments can be compared with EHT calculations. As an example, the experimental and calculated spectra of ethylene on Ni(l 11) are compared in Fig. 40. In the calculations, the model surface consisted of a Ni atom surrounded by six nearest neighbours in the surface plane and three in the plane below. The molecular plane of ethylene was taken to be parallel to the surface. 

In the past the theoretical model of the metal was constructed according to the above-mentioned rules, taking into account mainly the experimental results of the study of bulk properties (in the very beginning only electrical and heat conductivity were considered as typical properties of the metallic state). This model (one-, two-, or three-dimensional), represented by the electron gas in a constant or periodic potential, where additionally the influence of exchange and correlation has been taken into account, is still used even in the surface studies. This model was particularly successful in explaining the bulk properties of metals. However, the question still persists whether this model is applicable also for the case where the chemical reactivity of the transition metal surface has to be considered. 

Similar to macroscopic polymer networks, microgels have a more or less fixed surface. Due to the large value of their surface/mass ratio, microgels may be used as models for studying topochemical reactions at polymer surfaces. 

The dynamics of high-temperature CO adsorption and desorption over Pt-alumina was analyzed in detail using a transient mathematical model. The model combined the mechanism of CO adsorption and desorption (established from ultrahigh-vacuum studies over single-crystal or polycrystalline Pt surfaces) with extra- and intrapellet transport resistances. The numerical values of the parameters which characterize the surface processes were taken from the literature of clean surface studies ... 

This chapter deals with the study of structural properties of catalysts and catalytic model surfaces by means of interference effects in scattered radiation. X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. Low energy electron diffraction is the surface sensitive analog of XRD, which, however, is only applicable to single crystal surfaces. LEED reveals the structure of surfaces and of ordered adsorbate layers. Both XRD and LEED depend on the constructive interference of radiation that is scattered by relatively large parts of the sample. As a consequence, these techniques require long-range order. 

The importance of these radicals in catalytic processes may be evaluated by studying their behavior in stoichiometric reactions and by extrapolating this information to catalytic conditions. In following the stoichiometric reactions, magnesium oxide has been an excellent model surface since the three types of oxygen ions may be selectively formed and are stable at temperatures where most hydrocarbons of interest will react. Magnesium oxide, on the other hand, is basic and reactive itself therefore intermediates may react differently on this surface than on silica, for example. 

The examples introduced above refer to the characterization of the most common types of catalysts, usually supported metals or single, mixed, or supported metal oxides. Many other materials such as alloys [199,200], carbides [201-203], nitrides [204,205], and sulfides [206] are also frequently used in catalysis. Moreover, although modem surface science studies with model catalysts were only mentioned briefly toward the end of the review, this in no way suggests that these are of less significance. In fact, as the ultimate goal of catalyst characterization is to understand catalytic processes at a molecular level, surface studies on well-defined model catalysts is poised to be central in the future of the field [155,174], The reader is referred to the Chapter 10 in this book for more details on this topic. 

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